Although the 2005 Code-change cycle often included lively debate on the use of non-shielded cables at 4.16kV and 2.4kV, in the end, the majority of the members of Code-Making Panel 6 (CMP-6) voted to modify the exception to 310.6 (2002 version), which had allowed for the use of non-shielded cables at these voltage levels. The revised exception now allows for the use of 2.4kV non-shielded cables only if three conditions are met (Sidebar below). The end result of this change is that installing non-shielded cables to be operated on a 4.16kV system is now prohibited. Accordingly, Table 310.63 has been modified to reflect the new maximum operating voltage of 2.4kV.
Why the change? Some members of CMP-6 felt that non-shielded cables posed a significant threat to safety. Other members of the panel felt non-shielded cable is safe and effective when properly installed by trained and qualified personnel. Manufacturers of these cables presented data that indicated personnel could be at risk if they neglected safety considerations. In the end, the panel members decided it was best to limit the voltage level for non-shielded cables to 2.4kV, with exception.
The two most important concerns at 4.16kV are standing line voltage and line faults. Both can cause the outside of a non-shielded cable to develop a potential, which could pose a danger to anyone who touches it. One concern is that when a 5kV non-shielded cable is operated at 4.16kV, the surface of the cable isn't at zero potential. A standing voltage always exists and could, under certain conditions, be high enough to pose a danger to anyone who comes in contact with it.
A second concern is that if a non-shielded cable fails, the cable's jacket could become energized and the relay protecting the cable might not trip out of service. If a person were to grab a faulted cable, they could create a path to ground, which could easily result in injury or death. The conductive layer in the shield provides a return path to ground. If a fault does occur, relays can detect the current flow in that layer and de-energize the cable.
Functions of shielding. Shielding of an electric power cable is the practice of confining the electric field of the cable to the insulation of the conductor. This is accomplished by means of strand and insulation shields. A strand shield is employed to preclude excessive voltage stress on voids between conductor and insulation. To be effective, it must adhere to or remain in intimate contact with the insulation under all conditions.
An insulation shield has a number of functions that are meant to:
Confine the electric field within the cable.
Obtain symmetrical radial distribution of voltage stress within the dielectric, thereby minimizing the possibility of surface discharges by precluding excessive tangential and longitudinal stresses.
Protect cable connected to overhead lines or otherwise subject to induced potentials.
Limit radio interference.
Reduce the hazard of shock. If not grounded, the hazard of shock may be increased.
Where the insulation isn't protected by a metallic covering or shield, the electric field will be partly in the insulation and partly in whatever lies between the insulation and ground. The external field, if sufficiently intense in air, will generate surface discharge and convert atmospheric oxygen into ozone, which may be destructive to rubber insulation materials and protective jackets.
If the surface of the cable is separated from ground by a thin layer of air and the air gap is subjected to a voltage stress that exceeds the dielectric strength of air, a discharge will occur and cause ozone formation. The ground may be a metallic conduit, a damp non-metallic conduit, metallic binding tape or rings on an aerial cable, or a loose metallic sheath.
Damage to non-shielded cable may result when the surface of the cable is moist or covered with soot, soapy grease, or other conducting film, and the external field is partly confined by such conducting film so that the charging current is carried by the film to some spot where it can discharge to ground. The resultant intensity of discharge may be sufficient to cause the insulation or jacket material to burn.
How to properly ground shielded cable. To ensure positive continuity from one end of the cable to the other, the shielding of each cable should be grounded at both ends and at each splice. Where grounding conductors are part of the cable assembly, cable manufacturers recommend that the shielding be effectively grounded at both ends of the cable. For short lengths of cable, or where special bonding arrangements are used, grounding at only one point may be satisfactory. However, if operating conditions permit, it's desirable to bond and ground cable shields at more than one point to improve the reliability and safety of the circuit. This decreases the reactance to fault currents and increases the human safety factor.
The following single conductor cables that carry alternating current may, in general, be operated with multi-shield grounds:
Shielded cables up to and including 250 kcmil conductors with phases in separate ducts.
Shielded cables installed with all three phases in the same duct.
Cables of any size, provided allowance is made for heating due to current induced in the shield. Cables that carry direct current may always be solidly grounded at more than one point, except where insulating joints are required to isolate earth currents or to permit cathodic protection.
All grounding connections should be made to the cable shield in such a way as to provide a permanent low-resistance bond. A soldered connection is typically preferable to a mechanical clamp, because it presents less potential for a poor connection or loosening. The area of contact should be large enough to prevent the current from heating the connection and melting the solder. For additional security, a mechanical device, such as a nut and bolt, may be used to fasten the ends of the connection. This combination of a soldered and mechanical connection provides a permanent low-resistance path, which will maintain contact even if the solder happens to melt. Make sure the wire or strap used to connect the cable shield ground connection to the permanent ground is of ample size to carry fault currents.
Effects of grounding metallic shields. As previously noted, the metallic coverings of cables must be grounded to provide satisfactory operating and safety conditions. Because the method of grounding may affect the current-carrying capacity of the cable, apply the formulas that can be found in a manufacturer's handbook for calculating losses and correcting the current-carrying capacity for those losses.
Installation of shielded single-conductor cables must be studied to determine the best method of grounding, because voltage is induced in the shield of a single-conductor cable that carries alternating current caused by the mutual induction between its shield and any other conductors in its vicinity. This induced voltage can result in the following two conditions:
If metal shields are bonded or grounded at more than one point, circulating currents can flow through them. The magnitude of the currents depends on the mutual inductance to the other cables, the current in these conductors, and the resistance of the shield. This circulating current doesn't depend on the length of the cables or the number of bonds, provided there are bonds at each end of the cable. This circulating current heats the shield, which thereby reduces the effective current-carrying capacity of the cable. If the shield loss exceeds 5% of the copper loss, the current-carrying capacity of the cable should be reduced.
Shields bonded or grounded at only one point will create a voltage build-up along the shield. The magnitude of this voltage depends on the mutual inductance to the other cables, the current in all the conductors, and the distance to the grounded point. This voltage may cause unwanted electrical discharge or create an unsafe condition for a field worker. A safe potential is considered to be 25V for cables with a nonmetallic covering over the shield.
In the end, this change to 310.6 makes it clear that medium-voltage cables that operate at 4.16kV and higher must be shielded. The motivation for the change was the safety of personnel working in and around these cables. However, training of personnel is still important to the proper installation of medium-voltage cables — it just shouldn't be relied on as a replacement for shielded cables.
Owen is the owner and president of National Code Seminars in Pelham, Ala.
Sidebar: To Shield or Not to Shield?
As noted in 310.6 of the 2005 NEC, “Solid dielectric insulated conductors operated above 2000 volts in permanent installations shall have ozone-resistant insulation and shall be shielded. All metallic insulation shields shall be grounded through an effective grounding path meeting the requirements of 250.4(A)(5) or 250.4(B)(4). Shielding shall be for the purpose of confining the voltage stresses to the insulation.”
“Exception: Nonshielded insulated conductors listed by a qualified testing laboratory shall be permitted for use up to 2400 volts under the following conditions:
“(a) Conductors shall have insulation resistant to electric discharge and surface tracing, or the insulated conductors(s) shall be covered with a material resistant to ozone electric discharge, and surface tracking.
“(b) Where used in wet locations, insulated conductor(s) shall have an overall nonmetallic jacket or a continuous metallic sheath.
“(c) Insulation and jacket thickness shall be in accordance with Table 310.63.”
Sidebar: Concerns Related to Training
Some members of Code-Making Panel 6, as well as many other people, have concerns related to proper training for persons who install, troubleshoot, and work in and around medium-voltage cables. Some panel members felt that the solution in revising 310.6 of the 2005 NEC wasn't to omit the use of 4.16kV operating voltage, but to ensure that the persons working with these cables are properly trained. The benefit would be trouble-free installations, which are safe for personnel. The only problem with that concept is the concern over a declining level of training for this application. When training is provided, it can't be guaranteed that the training will be effective.